The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions.
Batteries can be broadly classified into primary and secondary batteries. Primary batteries, also referred to as disposable batteries, are intended to be used until depleted, after which they are simply replaced with one or more new batteries. Secondary batteries, more commonly referred to as rechargeable batteries, are capable of being repeatedly recharged and reused, therefore offering economic, environmental and ease-of-use benefits compared to a disposable battery.
Although rechargeable batteries offer a number of advantages over disposable batteries, this type of battery is not without certain drawbacks. In general, most of the disadvantages associated with rechargeable batteries are due to the battery chemistries employed, as these chemistries tend to be less stable than those used in primary cells. Due to these relatively unstable chemistries, secondary cells often require special handling during fabrication. Additionally, secondary cells such as lithium-ion cells tend to be more prone to thermal events, such as thermal runaway, than primary cells, thermal runaway occurring when an internal reaction rate increases to a point that more heat is being generated than can be safely dissipated, leading to a further increase in both reaction rate and heat generation. Eventually the amount of generated heat is great enough to lead to the combustion of the battery, which can lead to further combustion of materials in proximity to the battery. Thermal runaway may be initiated by a short circuit within the cell, improper cell use, physical abuse, manufacturing defects, or exposure of the cell to extreme external temperatures.
Thermal runaway is of major concern since a single incident can lead to significant property damage and, in some circumstances, bodily harm or loss of life. When a battery undergoes thermal runaway, it typically emits a large quantity of smoke, jets of flaming liquid electrolyte, and sufficient heat to lead to the combustion and destruction of materials in close proximity. If the cell undergoing thermal runaway is surrounded by one or more additional cells as is typical in a battery pack, then a single thermal runaway event can quickly lead to the thermal runaway of multiple cells which, in turn, can lead to much more extensive collateral damage. Regardless of whether a single cell or multiple cells are undergoing this phenomenon, if the initial fire is not extinguished immediately, subsequent fires may be caused that dramatically expand the degree of property damage. For example, the thermal runaway of a battery within an unattended laptop will likely result in not only the destruction of the laptop, but also at least partial destruction of its surroundings, e.g., home, office, car, laboratory, and the like. If the laptop is on-board an aircraft, for example within the cargo hold or a luggage compartment, the ensuing smoke and fire may lead to an emergency landing or, under more dire conditions, a crash landing. Similarly, the thermal runaway of one or more batteries within the battery pack of a hybrid or electric vehicle may destroy not only the car, but may lead to a collision if the car is being driven, or the destruction of its surroundings if the car is parked.
One approach to overcoming this problem is by reducing the risk of thermal runaway. For example, to prevent batteries from being shorted out during storage and/or handling, precautions can be taken to ensure that batteries are properly stored, for example by insulating the battery terminals and using specifically designed battery storage containers. Another approach to overcoming the thermal runaway problem is to develop new cell chemistries and/or modify existing cell chemistries. For example, research is currently underway to develop composite cathodes that are more tolerant of high charging potentials. Research is also underway to develop electrolyte additives that form more stable passivation layers on the electrodes. Although this research may lead to improved cell chemistries and cell designs, currently this research is only expected to reduce, not eliminate, the possibility of thermal runaway.
FIG. 1 is a cross-sectional view of a conventional cell and cap assembly commonly used with lithium ion batteries employing the 18650 form-factor. Battery 100 includes a cylindrical case 101, an electrode assembly 103, and a cap assembly 105. Case 101 is typically made of a metal, such as nickel-plated steel, that has been selected such that it will not react with the battery materials, e.g., the electrolyte, electrode assembly, etc. For an 18650 cell, case 101 is often referred to as a can as it is comprised of a cylinder and an integrated, i.e., seamless, bottom surface 102. Electrode assembly 103 is comprised of an anode sheet, a cathode sheet and an interposed separator, wound together in a spiral pattern often referred to as a ‘jelly-roll’. An anode electrode tab 107 connects the anode electrode of the wound electrode assembly to the negative terminal which, for an 18650 cell, is case 101. A cathode tab 109 connects the cathode electrode of the wound electrode assembly to the positive terminal via cap assembly 105. Typically battery 100 also includes a pair of insulators 111/113 located on either end of electrode assembly 103 to avoid short circuits between assembly 103 and case 101.
In a conventional cell, cap assembly 105 is a relatively complex assembly that includes multiple safety mechanisms. In cell 100, tab 109 is connected to assembly 105 via a current interrupt device (CID). The purpose of the CID is to break the electrical connection between the electrode assembly and the positive terminal if the pressure within the cell exceeds a predetermined level. Typically such a state of over pressure is indicative of cell overcharging or of the cell temperature increasing beyond the intended operating range of the cell, for example due to an extremely high external temperature or due to a failure within the battery or charging system. Although other CID configurations are known, in the illustrated cell the CID is comprised of a lower member 115 and an upper member 116. Members 115 and 116 are electrically connected, for example via crimping along their periphery. Lower member 115 includes multiple openings 117, thus insuring that any pressure changes within case 101 are immediately transmitted to upper CID member 116. The central region of upper CID member 116 is scored (not visible in FIG. 1) so that when the pressure within the cell exceeds the predetermined level, the scored portion of member 116 breaks free, thereby disrupting the continuity between the electrode assembly 103 and the battery terminal.
Under normal pressure conditions, lower CID member 115 is coupled by a weld 119 to electrode tab 109 and upper CID member 116 is coupled by a weld 121 to safety vent 123. In addition to disrupting the electrical connection to the electrode assembly during an over pressure event, the CID in conjunction with safety vent 123 are designed to allow the gas to escape the cell in a somewhat controlled manner. Safety vent 123 may include scoring 125 to promote the vent rupturing in the event of over pressure.
The periphery of CID members 115/116 are electrically isolated from the periphery of safety vent 123 by an insulating gasket 126. As a consequence, the only electrical connection between CID members 115/116 and safety vent 123 is through weld 121.
Safety vent 123 is coupled to battery terminal 127 via a positive temperature coefficient (PTC) current limiting element 129. PTC 129 is designed such that its resistance becomes very high when the current density exceeds a predetermined level, thereby limiting short circuit current flow. Cap assembly 105 further includes a second insulating gasket 131 that insulates the electrically conductive elements of the cap assembly from case 101. Cap assembly 105 is held in place within case 101 using crimped region 133.
Elements 115, 116 and 123 must be fabricated from a material that does not react with the electrolyte used in the electrode assembly. Accordingly, for a conventional lithium ion cell, these elements cannot be fabricated from steel. Typically they are fabricated from aluminum. In contrast, terminal 127 is generally fabricated from steel, thus allowing resistance welding to be used to attach a conductor to the terminal.
In a conventional cell, such as the cell shown in FIG. 1, a variety of different abusive operating/charging conditions and/or manufacturing defects may cause the cell to enter into thermal runaway, where the amount of internally generated heat is greater than that which can be effectively withdrawn. As a result, a large amount of thermal energy is rapidly released, heating the entire cell up to a temperature of 900° C. or more and causing the formation of localized hot spots where the temperature may exceed 1500° C. Accompanying this energy release is the release of gas, causing the gas pressure within the cell to increase.
To combat the effects of thermal runaway, a conventional cell will typically include a venting element within the cap assembly such as that previously shown and described. The purpose of the venting element is to release, in a somewhat controlled fashion, the gas generated during the thermal runaway event, thereby preventing the internal gas pressure of the cell from exceeding its predetermined operating range. Unfortunately in a conventional cell, the cell wall may still perforate (e.g., at site 141) due to the size of the vent, the material characteristics of the cell wall, and the flow of hot gas traveling along the cell wall as it rushes towards the ruptured vent. Once the cell wall is compromised, i.e., perforated, collateral damage can quickly escalate, due both to the unpredictable location of such a hot spot and due to the unpredictable manner in which such cell wall perforations grow and affect neighboring cells. For example, if the cell is one of a large array of cells comprising a battery pack, the jet of hot gas escaping the cell perforation may heat the adjacent cell to above its critical temperature, causing the adjacent cell to enter into thermal runaway. Accordingly, it will be appreciated that the perforation of the wall of one cell during thermal runaway can initiate a cascading reaction that can spread throughout the battery pack. Furthermore, even if the jet of hot gas escaping the cell perforation from the first cell does not initiate thermal runaway in the adjacent cell, it may still affect the health of the adjacent cell, for example by weakening the adjacent cell wall, thereby making the adjacent cell more susceptible to future failure.
One challenge to altering cell design when addressing these issues is that battery cell manufacturers produce enormous volumes of battery cells. Part of the manufacturing process includes “burn-in,” testing, and other operational functions that make use of existing processes and equipment. These processes and equipment depend upon a particular form factor for the battery cell. Even under circumstances when a customer is able to use a second form factor for the battery cell that can address the identified issues during in-field operation, adoption of the second form factor is problematic unless the manufacturer environment is considered and accounted for.
Accordingly, what is needed is a cell design that can help maintain cell wall integrity during a thermal event by efficiently allowing hot gas and debris to exit the cell via the cap. The present invention provides such a cell design.